A new class of high molecular weight organosilicon polymers and prepolymers which have excellent physical, thermal and electrical properties and outstanding resistance to water, and that can be used to prepare shaped articles is described by the instant inventor in U.S. patent application Ser. Nos. 07/079,740 (now U.S. Pat. No. 4,900,779) and 07/232,826 (now U.S. Pat. No. 4,902,731). They are thermoset or thermoplastic organosilicon polymers comprising alternating polycyclic hydrocarbon residues and cyclic polysiloxanes or tetrahedral siloxysilane residues linked through carbon to silicon bonds. This application is directed to novel organosilicon polymers and prepolymers, such as those described in U.S. patent application Ser. Nos. 07/079,740 now U.S. Pat. No. 4,900,779 and 07/232,826 now U.S. Pat. No. 4,902,731, further comprising linear, short chain .tbd.SiH terminated polysiloxanes.